Knocking, sensor or module?

[Guest]

Been trying to figure this out without luck and don't want to just throw parts at the engine. I get a knocking like pre-ignition from 3500 and up. It is getting worse. It does it under no load tied up to dock, does it under load, does it while cruising. I have already changed the plugs and cleaned the corrosion off the cap, a new one will be here in a day or two. I am certain it is not the cap at this point.

The system: 2004 Mercruiser 5.0 MPI pushing a Bravo 3 outdrive, engine serial number 0W029519 so that makes it an early MPI. 200 hours

Knock sensors are like $50 or so, I spent that much on 4 spark plugs. (spendy special buggers) Should I just get one and go for it? Is the torque as critical as manuals say? One other point, this pup is packed in with minimal room to access anything, the sensor will be hard to take out and a 100% by feel job.....I might get a camera setup ..............

This is now bad enough I don't want to use the boat and the dealer just doesn't have time to look at it yet.

[canoel]

Is this a pre ignition pinging, detonation or a knock?

Possibly the drive coupler or U-joints. Rarely will an engine exhibit a pre-ignition with no load.

Have you used a Rinda or other scan tool to diagnose the knock sensor, check the timing?

[Guest]

No I don't have a Rinda....not even sure what it is, will google it after this post.

The dealer convinced me to do a tune up. No change. (Plugs, cap, rotor) I randomly changed the knock sensor, not as expensive as an hour shop labor, so.

Not the problem. Do not understand this. 3500 RPM and up.

Timing is predone apparently by the system, there is no timing spec for this motor. (2004, 5.0, MPI)

It sounds like pre-ignition.

[88fourwinns]

More investigation needs to be done.

One thing that immediately comes to mind is fuel choice. Marine engines are under heavy load all the time. So my personal opinion is that it is better to run them on premium fuel, to allow for things that happen over time like carbon build up and less than perfect fuel system performance (ie fuel pressure, injector spray pattern, etc).

On old school engines, such as the 4.3 in my carbureted OMC, the manufacturer gives two different timing specs, one for use with regular fuel (1*BTDC) and one for premium fuel (6*BTDC). Modern engines of course do not have the option of changing the base timing, but that is why knock sensors came into use, to protect the engine if a lean fuel mix or carbon build up issue develops.

Things that can cause pre ignition are lean fuel mixtures and excessive carbon build up. Looking at the spark plugs can tell you a lot. If the center electrode insulator is blistered white that could be a sign of lean fuel . Causes on an engine with EFI could be a weak fuel pump, and partially clogged injectors. The center electrodes should be a light tan, or grey, not blistered white.

Checking compression will reveal carbon build up as seen by higher than normal comp test readings. Higher than normal compression test results if due to carbon build up will increase the octane requirement of the engine. Most of those engines should be between 150/175 for a hot reading with the throttle valve open. If they are a lot higher like 185+ that could be excessive carbon.

Lastly if it knocks with no load it well could be the U joints. One way to tell is with the boat out of the water remove the drive. Rig up a way to feed water to the raw water pump and start the engine . If it does not knock at the speeds noted before then it's likely to be the U joints or the outdrive. Even in neutral the upper gears turn when the engine is idling.

The easiest thing to try is to let the fuel level get low and then re-fill the tank with 93 octane gas. If the pinging stops then you need to look at fuel pressure, injectors and/or carbon build up. There is also a small chance that a vacuum leak is leaning out the fuel mix by introducing extra air that the fuel injection system cannot adjust for, but vacuum leaks usually cause other problems like a rough idle, odd hissing noises, etc.

[88fourwinns]

Also what is your hot slow idle oil pressure reading and engine water temp at idle and cruising on plane?

Chronically high cooling system temps + carbon build up + lean fuel mix could be a bad combination. If raw water cooled with a 160* stat, your engine should run between 160-175*.

[Guest]

Oil and fuel pressure are ok, don't have the numbers off hand. Temp runs 170 consistently. Boat runs great. Fuel has been 93 octane alcohol free.

The plugs looked correct when I changed them. Did not do compression test.

[88fourwinns]

Can you run it on land on the water hose and localize the noise more so? Try having someone listen in the back of the boat right next to the transom assembly or in the engine compartment but as far back as you can fit (lol). If its coming from the drive or u joints it will be more evident then.

[Nauti_Mike]

"88fourwinns" post=828750 wrote:
Can you run it on land on the water hose and localize the noise more so? Try having someone listen in the back of the boat right next to the transom assembly or in the engine compartment but as far back as you can fit (lol). If its coming from the drive or u joints it will be more evident then.

Caution as the OP said the noise does not start until 3500 rpm and above. This should not be done under no load and or on a hose.

[Guest]

I took the boat out of the slip for the season yesterday. Before I did I gently worked the throttle up, (out of gear) and cannot locate the exact place. It is harder to hear with the cover open.

Another description of the sound that it reminded me of, like a water pump that is failing and wobbling. Belts run true. I have not tried running it with the belt off.

It is time for winterizing, 200 hour maintenance, oil change, already did the tune up. The outdrive has to be pulled on this one and the yoke, ujoints, replace bellows, etc. I am pretty certain the gimbal bearing needs replaced as I get the rumble on turns.

All that being said, once that is done, I could put it back in the water and test it, as that pretty well would cover everything except something loose and the ECU. I will have open water all winter, just so long as I drain it out again I should be fine, but need to do the outdrive stuff while it is sorta warm in the days still.

Please any suggestion, no matter how far fetched, is worth listening to.

[Chief_Alen]

When you have the drive off and eyeballing the u - joints post a picture please.

To be crystal clear up to 3500 there is no " knocking " and continues till you reach wot.

[Guest]

Mr. Chef Alen, though I have no value of your opinions, you do have a valid point, I was not clear on characteristics of the noise. I shall elaborate.

It starts at 3500 and above. It will do it sitting in a slip, it will do it cruising. It does not happen under heavy load at those RPMs. It is not constant. Reducing throttle just slightly, or increasing slightly may make it stop. Sometimes it stops all by itself. I do not EVER run my boats at WOT. I have had the noise up as high as 3800. It is a velcro type crackly noise that sounds exactly like a car engine pinging. I cannot locate (yet) a specific area of the sound.

The only work the boat has had done in the last year was a raw water pump replacement. The noise just sorta started, it does seem to be getting more frequent, but no greater intensity.

Fuel is 93 octane alcohol free. I take the boat out of the slip to a land based gas station to get it. Spark plugs were light tan, proper color. Dist cap had corosion and condensate.

That does bring up one possible question.....does this have a mechanical spark advance also that could be sticking or causing the issue? According to my research timing is not setable on the MPI engine.

I hope I gave enough information. With my past auto mechanic experience I tend to assume folks know everything I do. Ask if you need to know more.

At this point, I don't think it is a timing issue unless mechanical advance. EVerything has been replaced or changed that could cause it. Even beginning to think just keep on truckin and just wait until it gets bad enough to locate. Maybe the winter maintainence will get it. Who knows.

[Nauti_Mike]

Since an increase or decrease in throttle may make it stop it may be slop in the drive train (worn u-joints of bad gimbal bearing), next time it happens try turning the wheel full lock to full lock and see if it changes sound. If it does that points to the u-joints and or gimbal bearing.

Engine pinging typically occurs more under load but you stated it does not happen under heavy load.

[Guest]

True, that is why this is so puzzling. Will not test the way you suggested, in the process of winterizing and getting ready to pull the OD for gimbal bearing (it does need, it rumbles on hard turn. May just replace the U-joints on general principal. Only a few bucks more and while I am there, maybe save some future work.

Anyone know if a high end professional car mechanics OBD thing will work on this? Or, is it proprietary? I have a buddy who has one

One suggestion I got today was weak springs on mechanical advance....it if has one. Haven't looked yet

[Nauti_Mike]

No OBD (on board diatonic) is for auto and not boats. Mercruiser has its own format which requires the Rinda tool mentioned earlier.

Which the drive off you can inspect the u-joints at a minimum.

If I recall your 2004 5.0MPI only has electronic advance and no mechanical advance.

[canoel]

Yes, only the Merc or Rinda style will interface correctly.

The advance is controlled by the ECM, no advance weights or springs.

You can use a good advance timing light and check your timing advance curve.

Use graph paper, start at idle check the advance, write it down. Increase rpm by 500 rpms check advance again write it down, etc etc all the way up. You should have full advance of no more than 30-32 degrees of total advance by 3200-3500 rpm.